1. Technical Field
The present invention relates to a method for manufacturing a flexible printed circuit board, and particularly to a method for manufacturing a multilayer flexible printed circuit board having different thicknesses in different areas.
2. Discussion of Related Art
Flexible printed circuit boards (FPCB) have been widely used in electronic products such as mobile phones, printing heads and hard disks. In these electronic products, some parts may move relative to a main body. In such a situation, FPCBs are applied to provide electrical connections and transmit signals between such parts and the main body due to their flexibility.
FIG. 34 shows a multilayer FPCB structure, which has different numbers of layers in different areas. In other words, there are thick areas and thin areas within the same FPCB. The thick area can have a higher circuit density, whilst the thin area exhibits higher flexibility.
FIGS. 29-34 show a process for manufacturing such a type of FPCB. As shown in FIGS. 29 and 30, a first copper clad laminate (CCL) 41, a binder layer 45 and a second CCL 42 are laminated. As is shown in FIG. 31, dry films 412, 422 are respectively applied on the first CCL 41 and the second CCL 42, and then, the dry films 412, 422 are exposed and developed. Because there is a cliff-like thickness difference between the first CCL 41 and the second CCL 42, a gap 46 is formed in the included angle at the base of the ‘cliff’.
As shown in FIG. 32, during an etching process, when the first CCL 41 and the second CCL 42 are immersed in an etching solution, the solution can seep into the gap 46 and react with the dielectric layers of the first CCL 41 and/or the second CCL 42. As a result, the dielectric layers may become unstable and peel from the first CCL 41 and/or the second CCL 42.
Referring to FIG. 33, a third CCL 43 and a fourth CCL 44 are respectively laminated with the first CCL 41 and the second CCL 42, to make another multilayer FPCB. Referring to FIG. 34, in order to electrically connect the copper layers of the third CCL 43, the first CCL 41, the second CCL 42, and the fourth CCL 44, a via hole 47 is defined so as to penetrate all the four CCLs. The via hole 47 can be made by drilling or by laser ablation. After the via hole 47 is formed, a conductive layer, e.g., a copper layer, is formed on a sidewall of the via hole 47 by electroless plating or electroplating. In the plating process, the dielectric layer of the second CCL 42 is exposed in a plating solution, thereby forming a number of copper lumps 48 thereon. These copper lumps 48 can pierce dry film that is applied onto the second CCL 42 in the next pattern-forming process, and the etching solution used for developing the dry film can react with dielectric layer or copper layer of second CCL 42 and result in a poor quality product.
In the aforementioned process for manufacturing multilayer FPCB that has different number of layers in different areas, a step structure between different CCLs can causes a series of quality problems. Therefore, a new process for manufacturing multilayer FPCB is desired to overcome the aforementioned quality problems.